Zimbabweite, a new alkali-lead arsenic tantalate from St Anns mine, Karoi district, Zimbabwe
Zimbabweite (Na,K) 2 PbAs 4 (Ta,Nb,Ti) 4 O 18 , a new mineral with trivalent arsenic, has been recognized at St Anns mine, southeast of Miami, Karoi district, Zimbabwe, in kaolinized pegmatite and dump material. The mineral is honey yellow-brown in large crystals, which are as much as 1 x 1 x 2 cm, and is clear pale yellow in thin fragments. Oxidation results in darkening of the color. There is one excellent cleavage, {010}. Zimbabweite has a Mohs hardness of 5 to 5.5, a white streak, adamantine luster, brittle fracture, is not magnetic, and does not fluoresce under either short wave or long wave ultraviolet light. d m = 6.20(3) and d c = 6.16 g/cm3. Optically, the mineral is biaxial (+), 2V Z = about 80°. Mean reflectances in air for an {010} = X-Z cleavage plate are : 589 nm--16.6 %, 470 nm--17.7 %, 546 nm--17.1 %, 650 nm--16.4 %. The indices of refraction determined by immersion methods are all greater than 2.10. Optic axis dispersion is very strong with ν > r, a = Z, b = Y, and c = X. The mineral is very pale yelow and is moderately pleochroic and X = pale yellow brown, Y = light reddish brown, and Z =reddish brown. Zimbabweite is insoluble in common acids or bases. A chemical analysis yielded, in weight percent, Ta 2 O 5 46.5, As 2 O 3 26.5, PbO 15.0, Nb 2 O 5 4.8, Na 2 O 3.1, K 2 O 1.5, TiO 2 1.4, BaO 0.4, UO 2 0.3, Bi 2 O 3 0.2, H 2 O (total) 0.19, SnO 2 0.1, F 0.04, SrO 0.02, total 100.05. A calculated formula is : (Na 1.51 K 0.48 Ba 0.04 ) Σ2.03 Pb 1.01 (As 4.03 Bi 0.01 ) Σ4.04 (Ta 3.17 Nb 0.55 Ti 0.26 U 0.02 Sn 0.01 ) Σ4.01 O 18 . Zimbabweite is orthorhombic, space group is Ccma or Cc2a , with a = 12.233(2)Å, b = 15.292(2)Å, c = 8.665(2)Å, V = 1621. 0(4) Å 3 , with Z = 4. No structural relationship between zimbabweite and any other tantalate minerals is apparent.